1. Field of the Invention
The invention concerns the sizing of paper in connection with its manufacture using a so called internal sizing technique. As a sizing chemical starch is used which is provided with special properties, whereby it better meets the requirements of the modern paper manufacture than the conventional internal sizing starches (wet-end starches).
2. Description of Related Arts
Starch is a natural polymer, which consists of glucose monomers bonded by 1,4-.alpha.-D-glucoside bonds to each other. Each glucose monomer contains three free hydroxyl groups capable of forming hydrogen bonds. When starch is solubilized in water, which is achieved by heating an aqueous starch slurry, i.e. by cooking, a viscous solution is produced, in which the viscous character results from hydrogen bonds between water and starch hydroxyl groups, and depends on the molecular size of the starch. When such a starch gel is dried, water is repelled from the spaces between the hydroxyl groups, and the hydrogen bonds are formed directly between the starch chains and the fibers forming the paper. This kind of hydrogen bond is stable and is responsible for the sizing property of starch.
Natural starch is weakly anionic by its nature, similarly to the fibers and fillers used in paper manufacture. Thus, when starch is added to the paper pulp, fixation of the starch to the fibers is negligible, and consequently in the filtration step connected with the sheet forming, i.e. in the dewatering step of the paper manufacture process, the natural starch is flushed away with water. The retention of starch is weak on the paper machine forming wire. For improving the retention, natural starch is chemically modified to expose cationic properties by bonding etherically thereto a quaternary nitrogen containing reagent. The cationicity of the cationic starches is expressed as a molar ratio between the substituted (cationic) glucose units and all the glucose units, i.e. as a degree of substitution.
Cationic internal sizing starches form the most frequently used group of chemical additives which increase the dry strength in paper manufacture. The starch used for the manufacture of the internal sizing starch may originate from potato, cereals or tapioca. The most commonly used raw material is potato starch. The use of internal sizing starches is disclosed for instance in the book of James P. Casey (Ed.), Pulp and Paper Chemistry and Chemical Technology, 3th Edition, Volume III, Chapter 14, Natural Products for Wet-End Addition, 1981, pp. 1475 to 1514.
The aim in modern paper manufacture is to increase the speeds of the paper machines. This leads to the requirement of providing good dewatering properties on the paper machine forming wire. Effective dewatering leads in turn to new requirements for the retention of fibers, fillers and internal sizing starches on the wire. The increase in the machine speed sets also greater demands on the paper strength. Especially the former-type paper machine wire parts, by which the dewatering efficiency has been attempted to be increased, put new demands on the strength of the paper in the z-direction. Starch has an important role in the strengthening of the paper in this direction.
As mentioned above, starch is a hydrocolloid with a water bonding capacity. On the other hand, a cationized starch is also a cationic polymer, which, due to the cationic character, has the property to increase dewatering so, that the higher the degree of cationicity the higher the dewatering. When starch is administered in the paper machine wet-end, the dewatering is increased at the beginning as long as the amounts added are smaller, but when the administered amounts are higher, the water binding capacity of the starch overrides the benefit received from the higher cationicity, and the dewatering capacity of the paper machine is decreased. This is what happens especially when lower cationic starches are used, whereby the dewatering often tends to limit the paper machine speed. The paper machine speed can be increased if the efficiency of the starch can be improved either so that smaller amounts are required or so that the water binding capacity of the starch can be lowered without affecting the strength properties.
One possibility to increase the dewatering properties of cationic starches is to increase the cationicity. As far as the cationicity is concerned, the present slurry processes are capable of achieving a degree of substitution of 0.05 without problems in the solubility of the starch. In order to achieve products with a higher cationic charge, dry cationizing processes are to be used. A disadvantage of these cationizing processes is the purity of the products, which is lower than the purity achieved by the slurry processes. Another feature, which limits the possibility to increase the degree of cationicity is the dosage of the starch which cannot be very high without the risk of a too high cationic charge in the stock flow, as easily happens for starches with a high cationic charge. At lower doses a lower strength must be accepted, as the strengthening effect is directly proportional to the starch content in the paper.
As an increase in the degree of cationicity does not as such lead to the intended result, another solution for increasing the efficiency of a cationic starch has to be found. One possibility is a further modification of the cationic starch. A common procedure in the manufacture of cationic surface sizes is oxidative degradation of the starch chains. Although this procedure would have positive influences on the dewatering on a paper machine, a decrease in starch retention would obviously be the result if applied on internal sizing, because the proportion of cationic groups in an individual starch molecule would be lower. In order to increase the retention, a higher degree of cationicity would be required. According to common knowledge, the starch molecules are to be maintained as intact as possible in an internal sizing starch. This pertains also to starch treatment in the paper mill, where a too vigorous cooking or pumping may degrade the starch chains.
Consequently, substantially the only possibility to try to change the properties of cationic starches is to increase the molecular size of the natural starch.
This approach is taken when crosslinked cationic starches are used in paper manufacture, U.S. Pat. No. 5,122,231 and EP-A1-0 603 727. Problems relating to the manufacture and use are, however, involved in these known methods. When using these modification processes, the crosslinking must be performed in a separate process step. The starch produced in this way also requires special cooking conditions in order to be in a suitable form for addition to the paper. The aforementioned citations do not disclose any information of manufacturing a cross-linked starch using dry cationizing methods.
Products for cationizing starches are commercially available, of which e.g. the product developed by Raisio Chemicals (henceforth "commercial cationizing chemical") is produced by using trimethylamine and epichlorhydrin. In connection with the development of this cationizing chemical it has been found, according to a special feature of the invention, that when the reactive trimethylamine also contains a mono- and/or dialkylamine, the resulting cationizing chemical has properties which increased significantly the viscosity of the starch in the cationizing step. As the viscosity of the starch is mainly dependent on molecular size, it can be assumed that the products thus obtained have a higher molecular size. It has also been recognized that these products can well resist cooking by direct steam, which presently is the most common procedure for dissolving starch in paper mills. When used as internal sizing agents, the cationized starches produced in this way have also proven to function effectively as retention agents, and they have a beneficial effect on dewatering.
However, the use of a mono- and dimethylamine causes problems in carrying out the invention. The produced cationizing chemical has a certain effect on the molecular size of starch, which effect results from these compounds. However, a cationizing chemical is primarily used for achieving a certain degree of cationicity in starch, i.e. a certain amount of cationizing chemical is used per certain amount of starch. In this way, the effects of the cationizing chemical on the degree of cationicity and on the molecular size are interdependent with each other. Alternatively, in order to achieve a certain degree of cationicity and a certain molecular size, a different cationizing chemical for each purpose should be prepared, in which the proportion of the epichlorhydrin to the amount of mono- and/or dimethylamine would be chosen according to the intended degree of cationicity and molecular size, respectively.